The present invention relates to an interferometric measuring device for detecting the shape, roughness or distance of surfaces by using a modulation interferometer having a spatially coherent beam source and a first beam splitter for splitting its beam into two partial beams, one of which is shifted in its light phase or light frequency with respect to the other by a modulation device and then the two partial beams are combined, having a measuring probe in which the combined partial beams are split into a measuring beam guided through a measuring arm and reflected on the surface and a reference beam guided through and reflected in a reference arm, and in which the reflected reference beam is superimposed on the reflected measuring beam, and having a receiving unit for splitting the superimposed beam into at least two beams having different wavelengths and converting the beams into electrical signals, and for analyzing the signals on the basis of a phase difference.
An interferometric measuring device is known from European Patent No. 126 475. In this known measuring device, rough surfaces of a measured object are measured interferometrically, a beam gun unit having laser light sources which emit light of different wavelengths being used. The laser light is divided into a reference beam of a reference beam path and a measuring beam of a measuring beam path using a beam splitter. The measuring beam path impinges on the surface to be measured, while the reference beam path is reflected on a reference surface, for example in the form of a mirror. The light reflected from the surface and the reference surface is combined in the beam splitter and focused, with the help of a lens, in an interferogram plane, where a speckle pattern is obtained. This speckle pattern is analyzed to determine the surface shape, a phase difference of the interferogram phases in the measuring point being determined. In order to simplify the analysis, a heterodyne method is used, the frequency of the reference beam being shifted with respect to the frequency of the measuring beam by a heterodyne frequency using a frequency shifter in the reference beam path. With this measuring device, a fine resolution of the surface shapes can be obtained.
However, the adjustment and handling are complicated for use in industrial manufacturing, for example.
Another interferometric measuring device is described in German Published Patent Application No. 39 06 118, in which optical fibers are provided between a plurality of laser light sources and a measuring section. Here again, a phase difference is evaluated for determining the surface structures. This known design is also disadvantageous with regard to the handling and adjustment in places that are difficult to access.
Another interferometric measuring device, as described in German Published Patent Application No. 198 08 273, which was not published previously, this device having a modulation interferometer and a spatially separate measuring probe connected to it by an optical fiber arrangement, is much more favorable for practical use in a manufacturing process for example. Advantages include in particular a short-term coherent radiation source of the modulation interferometer, yielding a stable beam that can be analyzed well, and the relatively compact design of the measuring probe. However, the arrangement of the measuring probe with respect to the surface of the measured object is still associated with some adjustment measures which would make a simplification seem desirable.
An object of the present invention is to provide an interferometric measuring device of the yielding simplified handling with a relatively simple, compact design.
Accordingly, the modulation interferometer designed as a basic unit which is spatially separate from the measuring probe and can be connected to it by an optical fiber arrangement, and the measuring arm and the reference arm are formed by solids conducting the measuring beam and the reference beam.
The measuring probe which is spatially separate from the modulation interferometer, which is designed as a basic unit, by the optical fiber arrangement is itself compact due to the design of the measuring arm and the reference arm as a solid and thus it is easy to handle and is designed to be easily adjustable with respect to the measured object. It is easy to adjust the reference arm and the measuring arm of the measuring probe relative to one another and with respect to the interferometric measuring device because of the unambiguous positioning of the individual elements.
If the design is such that a collimator device is provided at the input of the measuring probe and a focusing device is provided at the output of the measuring arm, and a deflecting element downstream from the focusing device is provided for output and then re-injection of the measuring beam directed at and reflected by the surface to be measured, then a favorable measuring beam and reference beam are obtained to form the interference pattern, and on the other hand the measuring beam can be aligned perpendicular to the measuring arm due to the deflecting element even when the surface to be measured is at an inclination to the direction of the measuring arm, so that a reliable measurement of the surface is achieved. An advantageous embodiment involves the collimator device and/or the focusing device being a GRIN lens.
Interference of the two partial beams before entering the measuring probe is prevented by the fact that one of the two partial beams in the modulation interferometer passes through a delay element which generates a difference in the optical path lengths of the two partial beams which is greater than the coherence length of the beam emitted by the short coherent beam source, and another difference in the optical path lengths is generated in the measuring arm with respect to the reference arm, compensating for the difference in optical path lengths generated by the delay element, and this interference comes about only after reflection at the surface or in the reference arm and thus coherence multiplexing is made possible.
Multiple sections of the surface to be measured or multiple separate surfaces can be measured rapidly and reliably without repositioning the measuring probe by the fact that the measuring arm has at least one additional deflecting element with which the measuring beam guided in the measuring arm is split and directed at another site on the surface to be measured, and the measuring beam reflected by this surface is injected back into the measuring arm. There are two different design possibilities here due to the fact that different optical path lengths can be preselected in the modulation interferometer by different interchangeable delay elements, and the compensating difference in the optical path lengths is formed by adjusting a reflecting or deflecting element of the reference arm or due to the separate reference arms assigned to the individual measuring beams split off in coordination with the different delay elements. If separate reference arms are provided, then the selected measuring site on the surface is obtained unambiguously by allocation to the corresponding delay element without further adjustment through coherence multiplexing. However, it is relatively simple to adjust the compensating optical path difference of the measuring arm with respect to the reference arm by the reflecting and deflecting element of the reference arm.
Two different design options for the measuring probe consist of the fact that the measuring arm and the reference arm(s) are designed as separate arms of the measuring probe in the manner of a Michelson interferometer (as illustrated in FIGS. 1 and 2) or in a common arm in the manner of a Fizeau interferometer (as illustrated in FIG. 3). If the measuring arm and the reference arm here are designed in a common arm of the measuring probe, this yields an especially compact design of the measuring probe, which can be used even under unfavorable space conditions.
A further simplification of the design of the measuring device is achieved due to the fact that the beam directed toward and away from the measuring probe is passed over a common monomode optical fiber arrangement, and the beam sent to the receiving unit is output from the optical fiber arrangement by an arm section. Coupling of the measuring probe to the modulation interferometer and the receiving unit can be accomplished easily by using plug connectors.